Certain embodiments of the present invention relate to a diagnostic ultrasound system which measures and images anatomical structures and their movements. More particularly, certain embodiments relate to methods and apparatus for generating and displaying strain rate spectrums associated with moving tissue structure.
Within the field of ultrasound imaging, physicians have become interested in using tissue strain and strain rate for clinical measurements. The term “strain” refers to a characteristic of the tissue being examined. For example, the strain associated with muscle tissue corresponds to a ratio of the muscle tissue's initial length and the change in muscle tissue length during a prescribed time interval. In ultrasound imaging, the rate of change of strain (i.e. strain rate) is typically visually presented to a physician as a colorized 2-dimensional image, where variations in color correspond to different strain rates. It has become apparent that the viability of a segment of the cardiac muscle is related to the amount of muscle strain and temporal behavior of the strain that is performed by, or imposed on the muscle segment. Also, it has been determined that malignant tumors may be detected based on the resistance to compression.
Doppler methods to measure velocities can be divided into two different categories. One method is spectral display and the other is color display. In the spectral method, the Doppler spectrum for a single location in the image is calculated by splitting the ultrasound signal in short-time overlapping windows and calculating the spectrum in each window. The time-varying spectrum is displayed in a frequency-time display with the spectrum magnitude coded as grayscale intensity or color. The color method, on the other hand, calculates the mean Doppler frequency for each point in the image and color encodes it for display. Only the color method has previously been applied to strain rate imaging. In its simplest form, the color represents the difference in mean Doppler frequency at two spatial locations separated by a small distance, divided by the distance. A problem with the color method is that it may be difficult to discern areas in the image that give correct strain rate values and areas that are affected by decorrelation and acoustical noise, since only the mean Doppler frequencies are used.
Reverberations are caused by multiple reflections within the tissue. The reverberations and noise can bias the velocity gradient estimated within the tissue due to correlation with a false or corrupted echo. Falsely increased, decreased or even reversed strain rate estimates may result.
One application of real-time strain rate imaging is in cardiology. The strain rate gives a direct and quantitative measure for the ability of the myocardium to contract and relax. By imaging along the myocardium from an apical view, the local strain rate component along the long axis of the heart can be measured. Measuring the local strain rate component gives information about the local shortening and lengthening of the heart wall. By imaging from the parasternal view, the strain rate component perpendicular to the heart wall gives information about the local thickening of the muscle. Wall thickening measured with M-mode or from the 2D image is a commonly used measure for muscle viability. With strain rate imaging, a direct measure for this thickening is available. The strain rate images can potentially add to the diagnosis of a number of cardiac disorders.
To understand strain rate in more detail, it is assumed that a segment of tissue of initial length LO may be stretched or compressed or lengthens or contracts to a different length L. The one-dimensional strain, defined as
                    ɛ        =                              L            -                          L              o                                            L            o                                              (        1        )            represents a dimensionless description of the change. If the length L is considered to be a function of time, L(t), the temporal derivative of the strain, the strain rate, can be found using the equation
                              ɛ          ·                =                  δɛ                      δ            ⁢                                                  ⁢            t                                              (        2        )            
If the velocity, v of every point in the object is known, an equivalent definition of the strain rate is
                              ɛ          ·                =                              δ            ⁢                                                  ⁢            v                                δ            ⁢                                                  ⁢            r                                              (        3        )            
The equations also provide a useful description of the deformation of the tissue segment. The strain rate measures the rate of the deformation of the segment. If the strain rate is zero, the shape of the segment is not changing. If the strain rate is positive, the length of the segment is increasing, and if the strain rate is negative, the length of the segment is decreasing.
U.S. Pat. No. 6,099,471 to Torp et al. is directed to a method and apparatus for real-time calculation and display of strain in ultrasound imaging. Ser. No. 09/432,061 to Torp et al. is directed to a method and apparatus for providing real-time calculation and display of tissue deformation in ultrasound imaging.
A need exists for an approach to easily visualize strain rates such that an improved indication of the quality of the strain rate estimates due to the presence or absence of reverberation and other sources of noise may be directly assessed and such that more overall strain rate detail is shown for a particular tissue location.